Unless you go to a different job every day, and return to a different home every night, you have a system. It's a collection of habits, routines, and tendencies that you follow to accomplish what you need to do, whether you're aware of it or not. realising this and figuring out how you accomplish work is the first step in figuring out how to make that system better:

If you’re yet to really consider your system, don’t ask yourself if you need one. Don’t start by asking yourself something along the lines of “do I need to read Getting Things Done?” Instead, start by asking this: is how I’m working, working? If the answer is yes, great! Get back to work. If not, don’t worry, but start looking to understand the system you’re pretending not to use and then [to] start figure out how to make it better.

If you accomplish a full week's worth of work without writing anything in a to-do list or managing a calendar, then your system may be working just fine. If, however, you find yourself repeatedly frustrated and need to adjust your habits, take a look at how you do things already before trying to adopt an entirely new system that may or may not work for you.